The invention is based on a pump assembly which comprises a vane pump, which, in particular, is to serve to supply operating cylinders of a hydromechanical transmission of a motor vehicle with a pressure fluid under high pressure, and a second hydraulic pump, the displacement elements of which are positively driven and which serves to supply a circuit having a low system pressure, in particular a lubricating-oil circuit of the motor vehicle, with the pressure fluid. The two hydraulic pumps therefore work with the same operating medium.
A pump assembly which comprises a vane pump and a second hydraulic pump whose displacement elements are positively driven has already been disclosed by EP 0 128 969 A1. In this publication, the oil flow of the vane pump serves to supply pressure medium to a power-assisted steering system. The second hydraulic pump is a radial piston pump, the oil flow of which serves a device for regulating the level of the vehicle. The two hydraulic pumps of the known pump assembly are located in two pressure-fluid circuits which only have the oil supply tank in common.
A vane pump generally has a suction region in which first pressure spaces between the vanes and second, rear pressure spaces behind the vanes become larger and receive pressure fluid in the process. In a pressure region, the pressure spaces become smaller, as a result of which pressure fluid is displaced to a pressure outlet. For satisfactory functioning of the vane pump, it is necessary for the vanes, which are guided in radial slots of a rotor, to bear against a stroke ring. Centrifugal forces which act on the vanes are utilized for such a unit, the effect of which centrifugal forces requires a substantial pressure balance between the front side bearing against the stroke ring and the rear side of the vanes in the slots. This condition is met due to the fact that the rear pressure spaces are also connected in the pressure region to the pressure outlet of the pump. In the suction region, both the first pressure spaces and the second pressure spaces are normally connected to the suction inlet of the vane pump, so that the same pressures again prevail in them.
The higher the viscosity of the pressure fluid, this viscosity increasing with decreasing temperature, the higher are the centrifugal forces which are required for bringing the vanes to bear against the stroke ring. This means that a vane pump of a conventional type of construction only begins to deliver at a rotational speed which is all the higher, the lower the temperature of the pressure fluid is. In particular, the engine and transmission oil of a motor vehicle, in particular of a farm tractor, may become so viscous at low ambient temperatures that the vane pump only begins to deliver at unacceptable high rotational speeds.
The object of the invention is to develop a pump assembly according to the preamble of patent claim 1 in such a way that satisfactory operation is possible even at low ambient temperatures and thus at a high viscosity of the pressure fluid.
According to the invention, this object is achieved in a pump assembly of the introductory mentioned-type in that the rear pressure spaces of the vane pump are connected in the suction region to the pressure outlet of the second hydraulic pump. Since the displacement elements of the second hydraulic pump are positively driven, the second hydraulic pump starts to deliver when it is driven, irrespective of the viscosity of the pressure fluid. The pressure building up at its pressure outlet is then also present in the rear pressure spaces of the vane pump and produces at the vanes a force which, in addition to the centrifugal force, presses the vanes radially outward against the stroke ring. The system pressure in the circuit which is supplied by the second hydraulic pump is relatively low and may be within the region of, for example, 5 bar. The frictional force between the vanes and the stroke ring therefore increases only slightly in the suction region of the vane pump, so that the wear on these parts continues to remain low.
DE-B 17 28 276 has certainly already disclosed a pump assembly which comprises two hydraulic pumps and in which the rear pressure spaces at the vanes of a first hydraulic pump formed as a vane pump are connected in their suction region to the pressure outlet of the second hydraulic pump. Here too, however, the second hydraulic pump is a vane pump which fails with highly viscous pressure fluid, so that the problem underlying the invention is not removed in the pump assembly disclosed by DE-B 17 28 276.
Thus, the vane pump is preferably one with a variable displacement volume, since the consumption of non-utilizable energy can thereby be reduced compared with a vane pump having a constant displacement volume. Since, in particular when used in motor vehicles, in addition to being economical with primary energy, it is very important that the individual components are inexpensive, the vane pump according to patent claim 3 is advantageously directly controlled and, upon reaching a set maximum pressure with its displacement volume, returns to such an extent that, at the maximum pressure, only the small quantity lost due to internal leakage is replaced. The power loss which then results from the product of the maximum pressure and the leakage quantity is slight, since the leakage quantity is slight.
The second hydraulic pump is advantageously a gear pump, in particular an internal gear pump without a filling piece, which gear pump works quietly, is favorable in production and can also be configured in its construction in such a way that it can be combined with the vane pump to form a construction unit without great outlay, as specified in patent claim 6.